A telephone system generally has a telephone central office or an exchange, and one or more subscriber telephone sets which are in each case connected to the telephone central office via a subscriber line. The subscriber telephone sets can be telephones or any other telephone equipment. The subscriber line generally has two conductors or two wires which are called “tip” and “ring”. The tip wire and the ring wire transport both alternating current signals and direct current signals. The subscriber telephone set, together with the associated tip and ring wires, is generally called a loop or subscriber loop (SL).
The telephone central office handles the switching of telephone signals between subscriber telephone sets. Industry standards such as, e.g. the Telcordia (Bellcore) Technical Reference TR-NWT-00057, Functional Criteria for Digital Loop Carrier (DLC) Systems, Issue Jan. 2, 1993, determine the electrical signal levels which are used for switching, connecting and signaling within the telephone system. If, for example, a telephone call to a particular subscriber telephone set is registered, the telephone central office must send signals to the telephone set in order to signal the incoming call. For this purpose, the telephone central office, using a ringing signal generator, sends out ringing signals which have the effect that the subscriber telephone set generates a bell signal. The bell signal can be a ringing bell, an electronic tone or any other audible or visible bell signal. The ringing signals are applied directly to the subscriber line by the telephone central office.
In the telephone central office of the telephone system, a line card, the ringing signal generator and a voltage supply for the line card and the ringing signal generator are usually arranged. The line card supplies a number of subscribers and is usually installed in a rack in the telephone central office. The line card has a subscriber line interface circuit (SLIC) for each subscriber, as an alternative, can also have an internal ringing signal generator for each subscriber, instead of an external ringing signal generator.
The subscriber line interface circuit is coupled to a respective subscriber line via associated connections and is connected to the subscriber via the subscriber line. As an alternative, the subscriber line interface circuit can also be located outside of and remote from the telephone central office in a private automatic branch exchange (PABX). The subscriber line interface circuit couples the analog subscriber line operated at a high voltage to the analog and digital circuits in the telephone central office, which are operated at low voltages. The subscriber line interface circuit usually supports the familiar “BORSHT” functions of battery feed, of overvoltage protection, of ringing signal generation, of signaling, of coding or PCM conversion, of the conversion between a two-wire transmission and a four-wire transmission (hybrid) and of the testing.
The subscriber loop consisting of the subscriber line and the subscriber telephone set has a loop resistance. The loop resistance (RL) is composed of a line resistance of the subscriber line and the load resistance of the subscribers connected to the subscriber line. The line resistance of the subscriber line depends on the length of the subscriber line or the distance between the telephone central office and the subscriber telephone set, and the load resistance depends on the number of subscriber telephone sets connected to the subscriber line (REN—Ringer Equivalent Number). If, in the case of a short subscriber line, the subscriber telephone set is physically close, e.g. only a few blocks of houses away from the telephone central office, the line resistance has a value of about 0 ohms (Ω). If, in the case of a long subscriber line, the subscriber telephone set is far distant, e.g. several kilometers, from the telephone central office and the subscriber line interface circuit, the line resistance is up to 930 ohms. With regard to the load connected to the subscriber line, the resistance or impedance occurring between the terminals of a subscriber telephone set should be about 1 REN in the on-hook state, in correspondence with industry standards, 1 REN corresponding to an impedance of 7 000 ohms at 20 Hz. Furthermore, it must be possible to operate the telephone system with up to five subscriber telephone sets (5 REN) or an impedance of 1 400 ohms at 20 Hz.
A ringing signal generator or a subscriber line interface circuit which has a ringing signal generator must be capable of supplying ringing signals to differently loaded subscriber lines which have an arbitrary length such as, e.g. a short length with low impedance or a long length with high impedance. In correspondence with the abovementioned Telcordia industry standard, the ringing signal must be applied to the subscriber line as an alternating voltage with a direct-voltage component. The direct-voltage component is preferably 49 volts, measured in the telephone central office. With regard to the alternating voltage, at least 40 V (rms) must be present across the maximum permissible load of 5 REN. In this case, a ringing signal current of about 29 mA flows. If the subscriber line is long, the maximum resistance of the subscriber line is 930 ohms according to the abovementioned standard and the telephone central office or, respectively, the ringing signal generator must then supply an alternating voltage of about 85 V (rms) at 20 Hz. If, however, the subscribers or, respectively, the load are connected directly to the telephone central office, no alternating voltage of 85 V (rms) but only of 43 V (rms) is required. If, however, the ringing signal generator feeds the subscriber line with an alternating voltage of 85 V (rms), this corresponds to a ringing signal current of 55 mA. This is a current which is greater by about 60% than the current which is actually needed for a load of 5 REN.
As an example, external ringing signal generators are frequently used in the US which are connected to the subscriber line via relays. In this arrangement, only one ringing signal generator is usually used for a number of subscriber lines. In the case of conventional solutions with an external ringing signal generator, however, power losses of the order of magnitude of up to 4 watts occur. Since network terminations (NT) in ISDN (Integrated Services Digital Network) or DLC (Digital Loop Carrier) systems are frequently remotely fed and must continue to maintain the telephone service during a mains voltage failure (lifeline support), the telephone service cannot be maintained by battery for any length of time in the case of a mains voltage failure with the usual external ringing signal generators.
However, to provide for as long as possible an operation of network terminations in the case of a mains voltage failure, resistances are connected into the feed line within the ringing signal generator which limit the current in the case of conventional ringing signal generators. By this means, a steep rise in the ringing signal current which is caused by a small loop resistance in connection with a constant high ringing signal voltage is prevented for example in the case of a short subscriber line and the ringing signal current is kept as low as possible for these short subscriber lines which can even represent a short circuit. A disadvantage of such ringing signal generators consists in that in order to supply a ringing signal voltage of 43 V (rms) even when a full load (5 REN) is connected to the subscriber line, the open-circuit voltage must be increased which, in turn, produces an increase in the power loss.
To solve the problem of increased open-circuit voltage, a host controller of a line card is normally used which periodically measures a line current in the subscriber line and determines from this whether the subscriber line is long or short. The host controller then programs the ringing signal generator in such a manner that the power loss is reduced.
A disadvantage of line cards which measure the line current in order to reduce the power loss consists in that it is not possible to control the power loss of the ringing signal generator accurately by means of the line current and, therefore, the power loss cannot be optimally reduced.
The conventional line cards which have ringing signal generators and subscriber line interface circuits have a voltage supply which usually supplies a voltage of the order of magnitude of 150 V to the line card with the technology used here. The voltage is designed for the worst case of loading on a ringing signal generator during the generation of a ringing signal as a result of which a line card has a high power loss in cases with less loading on the ringing signal generator, e.g. in the case of short subscriber lines and few subscriber telephone sets connected to the subscriber line.
Therefore, a further disadvantage of conventional line cards and ringing signal generators consists in that the power loss of a line card cannot be optimally controlled and reduced even with regard to the supply voltage of the line card.